In recent years, in a semiconductor inspection device, medical diagnosis/measurement instrument, or the like, as an inspection object is miniaturized, a signal to noise ratio (Signal to Noise Ratio: SNR) decreases in a detection signal that inspects and measures the object. It is required that a desired signal on which a shape, physicality, and the like of the inspection object are reflected is detected with high accuracy from such a detection signal of a low SNR. For example, a semiconductor inspection/measurement device is a device that irradiates laser, light, an electron beam, or the like onto a wafer being an inspection/measurement object, converts scattered light or a secondary electron from the wafer into an electrical signal by a detector such as photomultiplier or a scintillator, and inspects and measures shapes of foreign objects or patterns on the wafer through a preamplifier, a signal processing circuit, or a display.
In the semiconductor inspection/measurement device, along with the refinement of semiconductors, amount of laser, light, or an electron beam is narrowed down to thereby improve measurement resolving power. On the other hand, by narrowing down the dose, intensity of the desired signal becomes small relatively as compared to noise generated in a detector, an amplifier, or the like, and therefore it is difficult to detect the desired signal with high accuracy.
Therefore, a method is used, for example, by using randomness of noise, averaging detection signals, and detecting the desired signal with high accuracy. JP-A-2008-286736 (Patent Literature 1) describes, for example, “a signal that responds to amplitude of voltage or current of a certain input signal is set to a detection target. In a multichannel weak signal detection system that detects a plurality of response signals changing in time particularly, an input signal is time-division-multiplexed, conditions of multiplexing are optimized, and averaging-processing in two stages is performed with respect to the response signal, and thereby a weak signal is detected at a high SN ratio”.
Further, as a method for improving a signal detection ratio of an input signal with high intensity noise, a technique disclosed in JP-A-2002-221546 (Patent Literature 2) is used. Patent Literature 2 describes “an input signal is input to a nonlinear circuit 11, its output time series is memorized (13), and its power spectrum is calculated (15). Further, noise is generated (16), its intensity is raised gradually, synthesized with the input signal after raising its intensity gradually, and provided to the nonlinear circuit 11. The power spectrum is calculated in each noise intensity, a peak of each power spectrum is detected (18), and its peak value is acquired. Further, a maximum peak value is acquired and used as a period of a weak signal for acquiring the period of the maximum peak (20)”.